意外转换到ViewGroup: layout标签是RelativeLayout

Question

我正在做这个页面上的"创建第二个活动"教程我每件事都做得很仔细，但还是出错了。我不明白为什么会出现错误，特别是当我只是复制/粘贴他们告诉我复制/粘贴的代码时!

下面的代码行:

ViewGroup layout = (VeiwGroup)  findViewById(R.id.activity_display_message);

给了我这个错误:

unexpected cast to ViewGroup: layout tag was RelativeLayout.

嗯，我不知道那在说什么。我知道xml文件是一个相对布局。但我不知道为什么这是个问题，也不知道问题到底在哪里。

Answer 1

将ViewGroup更改为RelativeLayout

当你调用ViewGroup类的时候你缺少了RelativeLayout方法的一些方法，因为ViewGroup是一个父类，而RelativeLayout只是它的高级版本和其他一些类。下面是这两个类的区别

@RemoteView
public class RelativeLayout extends ViewGroup {
public static final int ABOVE = 2;
public static final int ALIGN_BASELINE = 4;
public static final int ALIGN_BOTTOM = 8;
public static final int ALIGN_END = 19;
public static final int ALIGN_LEFT = 5;
public static final int ALIGN_PARENT_BOTTOM = 12;
public static final int ALIGN_PARENT_END = 21;
public static final int ALIGN_PARENT_LEFT = 9;
public static final int ALIGN_PARENT_RIGHT = 11;
public static final int ALIGN_PARENT_START = 20;
public static final int ALIGN_PARENT_TOP = 10;
public static final int ALIGN_RIGHT = 7;
public static final int ALIGN_START = 18;
public static final int ALIGN_TOP = 6;
public static final int BELOW = 3;
public static final int CENTER_HORIZONTAL = 14;
public static final int CENTER_IN_PARENT = 13;
public static final int CENTER_VERTICAL = 15;
public static final int END_OF = 17;
public static final int LEFT_OF = 0;
public static final int RIGHT_OF = 1;
public static final int START_OF = 16;
public static final int TRUE = -1;
public RelativeLayout(Context context) {
    super((Context)null, (AttributeSet)null, 0, 0);
    throw new RuntimeException("Stub!");
}
public RelativeLayout(Context context, AttributeSet attrs) {
    super((Context)null, (AttributeSet)null, 0, 0);
    throw new RuntimeException("Stub!");
}
public RelativeLayout(Context context, AttributeSet attrs, int defStyleAttr) {
    super((Context)null, (AttributeSet)null, 0, 0);
    throw new RuntimeException("Stub!");
}
public RelativeLayout(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
    super((Context)null, (AttributeSet)null, 0, 0);
    throw new RuntimeException("Stub!");
}
public boolean shouldDelayChildPressedState() {
    throw new RuntimeException("Stub!");
}
public void setIgnoreGravity(int viewId) {
    throw new RuntimeException("Stub!");
}
public int getGravity() {
    throw new RuntimeException("Stub!");
}
public void setGravity(int gravity) {
    throw new RuntimeException("Stub!");
}
public void setHorizontalGravity(int horizontalGravity) {
    throw new RuntimeException("Stub!");
}
public void setVerticalGravity(int verticalGravity) {
    throw new RuntimeException("Stub!");
}
public int getBaseline() {
    throw new RuntimeException("Stub!");
}
public void requestLayout() {
    throw new RuntimeException("Stub!");
}
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    throw new RuntimeException("Stub!");
}
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    throw new RuntimeException("Stub!");
}
public RelativeLayout.LayoutParams generateLayoutParams(AttributeSet attrs) {
    throw new RuntimeException("Stub!");
}
protected android.view.ViewGroup.LayoutParams generateDefaultLayoutParams() {
    throw new RuntimeException("Stub!");
}
protected boolean checkLayoutParams(android.view.ViewGroup.LayoutParams p) {
    throw new RuntimeException("Stub!");
}
protected android.view.ViewGroup.LayoutParams generateLayoutParams(android.view.ViewGroup.LayoutParams lp) {
    throw new RuntimeException("Stub!");
}
public CharSequence getAccessibilityClassName() {
    throw new RuntimeException("Stub!");
}
public static class LayoutParams extends MarginLayoutParams {
    @ExportedProperty(
        category = "layout"
    )
    public boolean alignWithParent;
    public LayoutParams(Context c, AttributeSet attrs) {
        super((android.view.ViewGroup.LayoutParams)null);
        throw new RuntimeException("Stub!");
    }
    public LayoutParams(int w, int h) {
        super((android.view.ViewGroup.LayoutParams)null);
        throw new RuntimeException("Stub!");
    }
    public LayoutParams(android.view.ViewGroup.LayoutParams source) {
        super((android.view.ViewGroup.LayoutParams)null);
        throw new RuntimeException("Stub!");
    }
    public LayoutParams(MarginLayoutParams source) {
        super((android.view.ViewGroup.LayoutParams)null);
        throw new RuntimeException("Stub!");
    }
    public LayoutParams(RelativeLayout.LayoutParams source) {
        super((android.view.ViewGroup.LayoutParams)null);
        throw new RuntimeException("Stub!");
    }
    public String debug(String output) {
        throw new RuntimeException("Stub!");
    }
    public void addRule(int verb) {
        throw new RuntimeException("Stub!");
    }
    public void addRule(int verb, int subject) {
        throw new RuntimeException("Stub!");
    }
    public void removeRule(int verb) {
        throw new RuntimeException("Stub!");
    }
    public int getRule(int verb) {
        throw new RuntimeException("Stub!");
    }
    public int[] getRules() {
        throw new RuntimeException("Stub!");
    }
    public void resolveLayoutDirection(int layoutDirection) {
        throw new RuntimeException("Stub!");
    }
}
}

ViewGroup类

你可以看到你没有调用相对布局所需的一些方法，这就是你得到错误的原因。

Answer 2

在文章(你给的链接)你可以找到注释:

注意:以前版本的Android Studio生成的XML布局可能不包含android:id属性。调用findViewById()如果布局没有android:id属性，将会失败。如果在这种情况下，打开activity_display_message.xml并添加属性android:id="@+id/activity_display_message"的布局

元素。